Aiming at the problem that the installation space of the steering drive shaft of the double-cross shaft universal joint for automobiles is limited, it is difficult to meet the conditions of constant speed transmission, and the phase angle needs to be adjusted to reduce the speed fluctuation. The method of spatial geometric projection is used to establish a double-cross shaft universal joint. The rotation speed ratio and rotation angle equation of the steering transmission shaft of the knuckle bearing model are verified by the constant speed condition. The effect of the phase angle on the speed ratio is quantitatively analyzed. The results show that when the two transmission angles are equal and the phase angle is equal to the angle between the transmission surfaces, the output shaft and the input shaft can achieve constant speed transmission; the two transmission angles are not equal When , the double cross shaft universal joint drive shaft cannot achieve constant speed transmission, but when the phase angle is equal to the angle of the transmission surface, the speed difference between the output shaft and the input shaft is the smallest. Therefore, it can be concluded that when the phase angle of the transmission shaft is equal to the angle between the transmission surface and the direction is opposite, it is the optimal phase angle. Then, the optimal phase angle solution formula based on the hard point of the steering system is deduced, and the calculation program is compiled, and the calculation result of the program is verified by an example. The analysis conclusion and the programmed program have theoretical and practical significance for the design of automobile steering system and the transmission research of other types of universal joints. Key words phase angle constant velocity transmission cross shaft universal joint steering transmission shaft hard point. Influence Analysis of Phase Angle of Double Cross Universal Joint Steering Transmission Shaft Li Ning1, 2 Li Yourong1 Zhou Sizhu2 Zeng Yunyun2 ( 1 The Key Laboratory of Metallurgical Equipment and Control of Education Ministry, Wuhan University of Science and Technology, Wuhan 430081, China) ( 2 Institute of Strength and Vibration of Mechanical Structures, Yangtze University, Jing zhou 434023, China) Abstract Aiming at the problem of the installation space of double cross universal joint steering transmissionshaft is limited, it is difficult to meet the condition of constant speed drive, and need to adjust the phase angle to reduce the rotating speed fluctuation, the rotating speed ratio and rotating angle equations of steering transmission shaft are established by means of spatial geometric projection method, and the equation is verified by using the constant velocity condition. The influence of phase angle on rotating speed ratio are quantitatively analyzed, the results show that the input shaft and output shaft can realize constant speed drive while two transmissionangle are equal and the phase angle is equal to the transmission plane angle, and the double cross universal joint transmission shaft can't realize the constant speed drive while two transmission angles are not equal. However, the speed difference of input shaft and output shaft is the smallest while the phase angle is equal to the transmission plane angle. Therefore, this is the best phase angle when phase angle is equal to transmission plane angle and their phases are reversal. Then, the optimal solution formula of phase angle is deduced based on hard point of steering system and the computer program is compiled. The calculation result of the program is accurate by example verification. The conclusion and the program has theoretical and practical significance on system design of automobile steering and research on the transmission of other types of universal joint． The universal joint of the cross shaft can transmit torque while realizing the angle change. It has simple structure, low wear and tear, high transmission power, and the transmission angle between the main shaft and the driven shaft is relatively large. In the project, in order to achieve constant speed transmission, the double cross shaft universal joint must meet the following three conditions [1]: ① The input shaft, the intermediate shaft and the output shaft should be in the same plane; ② The fork heads of the forks at both ends of the intermediate shaft In the same plane; ③ The angle between the input shaft and the intermediate shaft is equal to the angle between the intermediate shaft and the output shaft. Due to the compact layout in the cockpit of the car, the installation space of the steering column and steering gear is more restricted, and the steering drive shaft is difficult to meet the above three constant-speed transmission conditions. Reference [2-3] studies the functional relationship between the input and output angular velocities of the double universal joint on the basis of a single cross-axis universal joint; Reference [4-5] calculates the speed of the Hooke universal joint at any angle of rotation. ratio and turning angle difference, and came to the conclusion that the unequal speed ratio is both asymmetric and unbalanced; Reference [6] established the motion equation of the double Hook universal joint, and deduced the middle of the double Hook universal joint. Theoretical formula for the relationship between the phase angle of the flange fork at both ends of the shaft and the transmission ratio. Adjusting the phase angle of the intermediate shaft is an effective measure to reduce the influence of rotational speed fluctuations, but the existing literature has not systematically studied the influence of phase angle on system rotational speed fluctuations and given the optimal phase angle conclusion. How to choose the optimal phase angle has become a key issue in the design of steering system. Therefore, it is necessary to conduct an in-depth analysis of the influence of the phase angle of the double cross-shaft universal joint steering transmission shaft on the steering stability.